An air bag device includes an inflatable air bag that is normally folded, and an igniter for inflating the air bag. In the air bag device for protecting a vehicle occupant in the driver's seat, an inflatable air bag is secured to a mounting positioned in the middle of a steering wheel, and is retained within a cover. Upon collision of the vehicle, gas is generated by the igniter, expanding the air bag. Expansion of the air bag ruptures and spreads the cover, allowing the air bag to be unfurled between the steering wheel and the occupants in the vehicle. It is desirable in the field of air bag devices to modulate the rate at which the air bag expands toward the occupant when the igniter is actuated, rather than allowing unrestrained expansion toward the occupant.
Examples of air bags devices with controlled rates of expansion are disclosed in U.S. Pat. No. 5,393,134 (corresponding to JP-A-6-305388 published in 1994) and U.S. Pat. No. 5,280,954 (corresponding to JP-A-6-234344 published in 1994), wherein first and second panels of the air bag are stitched together, and the stitches or seams thus made are successively torn apart due to the pressure of the gas within the air bag, whereby expansion of the air bag is slowed. Other examples are disclosed in U.S. Pat. No. 5,308,113 (corresponding to JP-A-6-191366 published in 1994), U.S. Pat. No. 5,454,595, and JP-A-6-286543 published in 1994, and others, wherein the first and second panels of the air bag are connected by means of tethers, each of which are shortened by folding. In operation, stitches joining the tether folds are successively torn apart due to the pressure of the gas expanding the air bag, thereby limiting the rate the air bag is expanded. A further example is disclosed in JP-A-6-298025 published in 1994, wherein the first and second panels of the air bag are bonded together by a thermoplastic adhesive, such as a hot melt adhesive or a thermosetting adhesive, which is gradually melted, altered or degraded by the heat of the gas generated by the igniter, thereby limiting the rate the air bag is expanded.
FIG. 12 is a perspective view showing an inflated state of a known air bag for the driver's seat, and FIG. 13 is a cross sectional view showing tethers, as described above, used in the known air bag device. The air bag 10 shown in FIG. 12 has a first panel 11 and a second panel 12, each consisting of a circular cloth. The panels 11,12 are sewed together at a peripheral seam 15. The second panel 12 has an opening 14 formed in a middle portion thereof, through which an igniter 34 (FIG. 13) extends. Around the opening 14, there are formed holes 16 into which are inserted bolts, pins or rivets 31 used to secure the air bag 10 on a mounting 30 (FIG. 13). The second panel 12 further has a vent hole 18 adapted to release the gas generated by the igniter and stored in the air bag 10.
As shown in FIG. 13, the inflatable air bag 10 is secured by bolts, pins or rivets 31 to the mounting 30, and is retained within a cover 32. The first panel 11 and the second panel 12 are connected to each other, by tethers 22 that are shortened by folding to create tether folds 26. The tether folds 26 are stitched together with sewing threads 24. When the air bag 10 is inflated by gas generated in the igniter 34, the cover 32 is initially ruptured, and a central portion of the first panel 11 is projected outward through an opening 33 of the cover 32 while the peripheral seam 15 of the air bag 10 pushes the cover 32 sideways and progressively opens the cover 32. The air bag 10 is unfurled at a relatively low rate, since the peripheral seam 15 at the maximum diameter of the air bag 10 pushes open the cover 32 while being pressed against the inside wall of the cover 32 until the peripheral seam 15 reaches the opening 33 of the cover 32. Then, the air bag 10 is rapidly unfurled once the peripheral seam 15 of the air bag 10 is ejected out of the opening 33 in the cover 32 since the friction between the peripheral seam 15 and the inside wall of the cover 32 has been eliminated. Thus, the whole air bag 10 is deployed outside the cover 32, as shown in FIG. 13. At this point in time, the air bag 10 is prevented from being pushed toward the occupant (i.e., upward in FIG. 13) because the first and second panels 11,12 are connected by the tethers 22 with the tether folds 26. Thereafter, the gas pressure generated by the igniter 34 is directed at the first panel 11 so as to push the first panel 11 toward the occupant. Subsequently, the sewing threads 24 maintaining the tether folds 26 are broken enabling the tethers 22 to extend to their full length, whereby the first panel 11 is deployed toward the vehicle occupants.
In the air bag having the tethers 22 shown in FIG. 13, and other known air bags in which the front and rear panels are sewn or adhesively bonded to each other, the peripheral seam 15 having the maximum diameter is pressed against the inside of the cover 32 with the resulting friction between the air bag 10 and the inner wall of the cover 32 slowing the rate at which the air bag 10 is unfurled. Once this peripheral seam 15 is ejected outside of the cover 32, however, the resistance to unfurling due to the above-described friction is eliminated, and the remainder of expansion by the air bag 10 is unrestrained, exerting strong tensile forces on local parts of the air bag. Therefore, the panels 11,12 of the air bag 10 are required to have a thickness large enough to resist the localized tensile force.